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Finally I smiled at her and she returned a smile. Testing this, the same thing happened twice more. Just wondering if this is worth pursuing or not.

It's worth coming back to the store again.
I wouldn't escalate just because a chick smiled at you. You need a little more than this. Or you risk making a clumsy or an obvious move and being labeled as a problem customer. Or she'll close down completely and avoid you (this is related to ASD).
Go back shopping at the store sometime, and try to make some eye contact with her, or better yet use proximity (look at an item near where she is to try to get her to notice you, and see if she opens you or remembers you). Don't approach her, let her come to you. And if she does it's a positive sign and you should try to bait her into giving you IOI's (playing with hair, facing you, hanging around while you shop, etc..). Then you can escalate by asking her what's she's doing later, n-close, etc...
Sometimes these chicks will set you up if you play along. For instance, find something's that's not in your size and have her order it for you from another store. Then tell her to call you when it comes in. Then when she calls you, chat her up. If she balks, tell her you changed your mind about the item. Sneaky I know, but it works.
These retail situations can be somewhat complicated. If you play it right and discreetly retail can be an almost endless supply of hot girls. You mess it up once and you're done.
Word travels fast, and like all girls they look out for one other. Whether it's a fact is secondary to whether it is a female perception. Not to mention the problems you generally have from male managers in the stores themselves. They tend to watch these girls and their interactions like hawks. Part of the reason is they are paying them, the other reason is often they are trying to get with 'em too.
So be discreet and don't be obvious. If you can't do either or the situation isn't favorable for doing so (busy, etc..), better to come back to fight another day.